mac80211: remove beacon counters

The beacon counters mac80211 keeps are only used for debugfs,
unfortunately, they are incorrect for many hardware designs,
namely any design that has a beacon template. Hence, remove the
counters so we don't create the impression they are usable.

This also allows removing the beacon MESH #ifdef again.

Signed-off-by: Johannes Berg <johannes@sipsolutions.net>
Signed-off-by: John W. Linville <linville@tuxdriver.com>